Stereophonic electronic musical instrument



T aci l Sept. 19, 1967 HENLEY 3,342,923

STEREOPHONIC ELECTRONIC MUSICAL INSTRUMENT Filed Sept. 5, 1964 2Sheets-Sheet 1 $1 J M A g.

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STEREOPHONIC ELECTRONIC MUSICAL INSTRUMENT Filed Sept. 3, 1964 2Sheets-Sheet 2 s TAl/VG INVENTOR. fan/4,90 J. flEA/za United StatesPatent 3,342,923 STEREOPHONIC ELECTRONIC MUSICAL INSTRUMENT Edward J.Henley, Fanwood, N.J., assignor to M. P. Moller Incorporated,Hagerstown, Md., a corporation of Maryland Filed Sept. 3, 1964, Ser. No.394,142 13 Claims. (Cl. 84-1.24)

ABSTRACT OF THE DISCLOSURE A stereophonic electronic musical instrument,such as an electronic organ, wherein an attenuative bus is employed tocollect the signal outputs of the several oscillators employed in theinstrument and to distribute these signals to a plurality of amplifierand speaker systems. The speakers are spatially arranged, and both thesources of oscillations and the inputs of the amplifiers are connectedat spaced intervals along the attenuative bus, so that the accousticoutput of the instrument has a spatial distribution similar to that of apipe organ or other acoustic instrument.

My invention relates to electronic musical instruments and, moreparticularly, to elecrtonic organs.

Electronic musical instruments, such as electronic organs, have certainadvantages over more traditional instruments, such as pipe organs. Theseadvantages include low cost, great tonal variety, wide dynamic range,small size, and ease of construction. However, none of the electronicinstruments produces the ensemble effect of a large pipe organ.

In a pipe organ, each type of tone is produced by a separate set ofpipes, more commonly known as a rank of pipes. Even a moderately sizedpipe organ has several ranks of pipes. With each rank having up to 97pipes, the total number of pipes can readily exceed one thousand. Thesepipes are generally played from a console having two or more manualkeyboards and one pedal keyboard. By means of coupler switches, it ispossible to activate several pipes in each of several ranks bydepressing a single key at the organ console.

It is evident that a plurality of organ pipes do not function as a pointsource of sound. Indeed, the charm of a pipe organ lies largely in thefact that the pipes are spread out over many square yards of area, eachpipe sounding from its own particular location in space. The differencesin time of arrival of the sounds from many pipes, as well as the manyangles of arrival of direct and reflected sounds, help the listener toperceive that it is indeed a chorus of many independent sounds ratherthan a very limited number of tones radiated from a relatively fewpoints in space. It is this same spatial distribution of independenttones which distinguishes a chorus from a quartet and a symphonyorchestra from a small dance orchestra.

Many ingenious methods have been used in electronic organs to mask theirdeficiencies. These include several types of multiple speakerarrangements, such as a plurality of speakers radiating the same sounds,woofer-tweeter combinations wherein the sound is radiated from eachspeaker in accordance with the frequency response characteristic of thatspeaker and the associated dividing network, separate speakers for eachtonal quality or group 3,342,923 Patented Sept. 19, 1967 of similartonal qualities, and multiple channel arrangements in which, forexample, odd-numbered notes are radiated from one speaker system andeven-numbered notes are radiated from another. These may be classed aspseudo-stereophonic systems. Another such system is one in which bothchannels carry the same tones but one channel introduces a delay of manymilliseconds. Still other systems employ speakers in motion or systemsof sound dispersing bafiles to achieve similar effects. However, none ofthe systems employed to date has produced an ensemble effect comparableto that produced by a pipe organ with a similar tonal design.

Accordingly, it is an object of my invention to provide a multiplicityof independent sound sources in an electronic musical instrument.

It is also an object of my invention to provide a spatial distributionof tones comparable to that produced by a pipe organ.

It is a further object of my invention to reduce intermodulationdistortion in an electronic musical instrument.

Another object is to simplify the problem of voicing and regulating thevarious frequency ranges in an electronic musical instrument.

Still another object is to permit variation of the dynamic level in amanner that will compensate for characteristics of the human car.

A further object of my invention is to achieve high reliability byproviding enough independent elements in an electronic musicalinstrument whereby a breakdown in any element will not seriously impairthe over-all capacity of the instrument.

These and still further objects of my invention will become readilyapparent from the following detailed description, accompanying drawings,and appended claims.

In the drawings:

FIGURE 1 is a simplified diagram showing the essential features of myinvention.

FIGURE 2 is a diagram of a keyed oscillator which may be employed in anelectronic musical instrument.

FIGURE 3 is a diagram of a continuously operative oscillator withprovision for keying the audio output.

FIGURE 4 is a diagram of an oscillator showing an output path containingthe combination of two signals taken from two different points in theoscillator circuit.

FIGURE 5 is a typical front view of four sets of three loudspeakerseach, such as would be used in a small organ containing fourcombinations of the apparatus illustrated in FIG. 1.

For purposes of illustration, only the essential features of myinvention are shown in FIG. 1 to facilitate an understanding of theoperation and novel characteristics. Those skilled in the art recognizethat a complete instrument may contain a large number of such assembliesunder control of a single console. Various power and key switcharrangements are all well known and are not a part of this invention.Therefore, they have been shown only to the extent necessary for anunderstanding of this invention.

stereophonic reproduction of sound has been employed for many years, inthe most popular form, uses -a two channel system. More sophisticatedsystems employ three channels in order to eliminate the hole in themiddle which occurs when wide separations are used. However, certaintechnical problems have hindered development of a true stereophonicmusical instrument which produces sounds having the desired spatialdistribution. One of these problems is the precedence effect whichcauses the nearer of two sources of the same sound to appear as the truesource even though the farther source generates more acoustical power. Apower difference of twenty decibels or more is required to adequatelyminimize this limitation. My invention resides in a design employingthree or more channels in a manner which successfully minimizes theprecedence effect and has many other desirable features Which cannot beachieved in a two channel system. These results are not achieved simplybecause three channels are employed instead of two, but are due to themethod disclosed in this invention. This is analogous to a comparison ofa two-legged stool and a three-legged stool. A threelegged stool is notnecessarily self supporting, but it can be if the three legs arearranged in other than a straight line. The system employing three ormore channels, as disclosed in this invention, is the first everproposed to my knowledge which provides true stereophonic generation andradiation of musical tones with results comparable to those produced bya pipe organ.

Referring now to FIG. 1 of the drawings, a series of seventy-threeoscillators is used to generate tones corresponding to each key on oneof the instruments keyboards plus an octave extension of twelve tones.This is in accordance with well-known practice and it is understood thata greater or lesser number of oscillators may be employed, dependingupon the design requirements of the. particular instrument.

The output 11 of each of the seventy-three oscillators 10 is connectedthrough an isolating resistor 12 to an attenuative collector bus 13comprising a like number, less one, or seventy-two series-connectedresistors 14. The output of the first oscillator 10, shown at the leftof FIG. 1, is connected to one end of the attenuative collector bus 13,the output of the last oscillator 10, shown at the right of FIG. 1, isconnected to the other end of the attenuative collector bus '13, and theoutput of each intermediate oscillator 10 is connected to a junction oftwo of the series-connected resistors 14, so that each oscillator outputconnection is separated from the next oscillator output connection byone of the series-connected resistors 14.

Since the series-connected resistors 14 do not, in themselves, providethe required attenuation in the collector bus 13, three terminatingresistors 15, 16 and 17 are provided. Resistor 15 terminates one end ofthe bus 13, resistor 17 terminates the other end of the bus and resistor16 terminates the center of the bus \13. Termination of the bus 13 at anintermediate point to increase the total attenuation from end-to-end isvery important in order to obtain an adequate stereophonic effect.

Bridged across each of the terminating resistors 15, 16 and 17 is theinput of an amplifier 18. A loudspeaker 19 is connected to the output ofeach amplifier 1,8. In accordance with common practice, volume changesof the amplifiers 18 are accomplished by a ganged control 20. Thespecific methods for doing this are well known in the art and may, forexample, be mechanically ganged potentiometers or remotely controlledcircuits employing lamps and light-sensitive resistors in attenuativecircuits. However, the employment of three or more channels according tomy invention permits new and novel effects to be achieved as I shallpresently disclose.

FIG. 2 shows a transistor oscillator circuit which is suitable forproduction of one type of tone found useful in an electronic organ. Inthis circuit, the oscillator 10 is normally inoperative and is madeoperative upon closure of a key switch 21 which applies operatingpotential from a battery 22. It is understood, of course, that in acomplete electronic organ, the operating potential to operate alloscillators 10 is obtained from a common supply. The oscillator outputconnection 11 leading to the isolating resistor 12, shown in FIG. 1, isat point 23. The key switch 21 may be directly actuated by operation ofthe console keys or it may be a remotely controlled electro-mechanicalor electronic circuit, such as a diode gating circuit.

FIG. 3 shows a transistor oscillator 10, similar to the one shown inFIG. 2, except that the oscillator is normally operating and its outputconnection 11 is established as required by an audio key switch 24. Asin FIG. 2, this switch may be directly actuated or it may be one of themany well-known remotely controlled electronic switching circuits.

FIG. 4 shows a transistor oscillator circuit, similar to that shown inFIG. 2, except that the signal developed across a load resistor 25 atpoint 26 is combined in an out-of-phase relationship with the signaldeveloped at point 27 because of the phase reversal occurring in thecircuit. An additional isolating resistor 12 is provided to minimizeloading effects and a capacitor 28 is provided to block the directcurrent in the transistor collector circuit from getting into the signalcircuit. The output connection 11 at point 29 leads to the isolatingresistor 12, shown in FIG. 1.

It is obvious from the above that a wide variety of oscillator circuits,including systems employing master oscillators and cascaded frequencydividers, may be used without departing from the scope of my invention.

FIG. 5 shows a four cabinet array of loudspeakers 19 in an arrangementsuch as might be used in a small organ. By employing modular techniques,with three loudspeakers 19 in each horizontal cabinet 30, it is possibleto achieve great flexibility in the design of an organ. The array shownis part of an organ employing four combinations of loudspeaker equipmentshown in FIG. 1. Each loudspeaker combination is designed to produce adifferent tone quality. By placing the loudspeaker combination carryingthe strongest fundamental tones near the floor, it is possible to securean enhancement because of the increased radiation impedance at thejunction of the floor and the speaker cabinets 30. Each of the fourcabinets 30 contains internal partitions (not shown) to separate it intothree isolated speaker compartments. This is necessary to secure theacoustic isolation vital to a good stereophonic system.

Referring again to FIG. 1, it is apparent that the signal appearing atpoint 31 from the left oscillator 10, as viewed in FIG. 1, will bereproduced Without attenuation from having to pass along the collectorbus 13. However, the signal appearing at point 32 will have beenattenuated because of the voltage drop across resistors 14 betweenpoints 31 and 32. Resistor 16, paralleled by the seriesconnectedresistors between point 32 and ground 33, including terminating resistor17, presents a relatively low impedance at point 32 with a resultingsubstantial attenuation of the signal from the left oscillator 10, asviewed in FIG. 1. At point 34 the signal from the left oscillator 10 isstill further attenuated because of the voltage drop across theseries-connected resistors 14 between points 32 and 34, terminated byresistor 17.

In order to facilitate an understanding of further descriptions of thecircuit, I list below a set of circuit values which have yieldedexcellent results.

Ohms Isolating resistors 12, each at 100,000 Series connected resistors14, each at 47 Terminating resistors 15 and 17, each at 1,000Terminating resistor 16 1,500

Using 10% tolerance resistors with the above nominal values, theperformance is entirely satisfactory. It is to be noted that theimpedance of the bus 13 is not uniform along its length. This isadvantageous and helps to equalize the amplitudes of the radiatedsignals derived from the various oscillators 10. The attenuation rangesfrom 0.2 decibel to 0.5 decibel per section depending upon the pointsbetween which the measurements are made. If a signal from the leftoscillator 10 is applied to the bus 13, it is radiated with referenceamplitude by the left loud- Go speaker 19, as viewed in FIG. 1. The samesignal is radiated by the middle loudspeaker 19 approximately 11.5decibels below reference amplitude and by the right loudspeaker 19approximately 22 decibels below reference amplitude. It is evident thenthat most of the sound is raddiated from the left loudspeaker 19 and theapparent source of the sound is in close approximation to the positionof that loudspeaker. Those skilled in the art will, now appreciate thatwith the circuit shown, the distribution of energy, and therefore theapparent source of sound, will shift in accordance with which oscillatoris furnishing the signal. For example, if an oscillator approximatelymidway between the left oscillator 10 and the middle oscillator 10 atpoint 32 of FIG. 1 is furnishing the signal, the left and middleloudspeakers 19 will each radiate sound approximately 4 decibels belowthe previously mentioned reference amplitude and the right loudspeaker19 will radiate sound approximately 14.5 decibels below referenceamplitude. In this case, the apparent source of sound is close to midwaybetween the left and middle loudspeakers 19. If the oscillator 10 atpoint 32 is furnishing the signal, the middle loudspeaker 19 willradiate sound at reference amplitude and the two end loudspeakers 19will each radiate sound approximately 10.5 decibels below referenceamplitude. In this case the apparent source of sound will be the middleloudspeaker 19. In a similar manner, the apparent source of sound willcontinue to shift toward the right loudspeaker 19 as the oscillators 10furnishing the signals approach to the right,

that is, from point 32 towards point 34. This action takes place whetherthe oscillators are operated singly or in combination.

In the examples given, it has been assumed that the amplifiers 18 haveequal gains. However, it is evident that an arrangement according to myinvention lends itself to shaping of the volume characteristic accordingto the fundamental frequencies rather than according to the frequencycomponents. For example, if the oscillators 10 are connected to the bus13 in a chromatic progression with the lowest frequency oscillator 10 atone end and the highest frequency oscillator 10 at the other end, thevolume of the tones from the higher frequency oscillators can be reducedby reducing the gain of the appropriate amplifier without affecting tonequality. If conventional tone control methods were applied, the higherstring-type tones would begin to sound like flutes.

The three-channel arrangement which I prefer, having the oscillators 10connected to the bus 13 in a chromatic progression, lends itself toanother desirable variation. By making the controllable volume range ofthe amplifiers unequal and gauging the controls so that they areoperated simultaneously by a foot-operated lever, it is possible tocompensate for limitations of the human ear and to maintain a tonalbalance over a wide dynamic range of volume levels. For example, theamplifier which favors the output of oscillators operating in the middleof the audio range can be made to have a thirty decibel range of gainadjustment while the other two may have only a twenty or twenty-fivedecibel range covered simultaneously by a ganged control.

In addition to permitting variable control of volumes over the frequencyrange, the three-channel arrangement is obviously adaptable toregulation of the tone color through use of conventional tone controlsor formant filters in the individual amplifiers. Typical circuits arenot shown because these are so well-known in the art. However, thestereophonic system according to my invention is the first which permitsuse of multiple formant filters which affect different frequency rangeswithout undesirable interaction. This is because the filters used ineach amplifier can have characteristics differing from those of thefilters used in the other amplifiers. The tone quality transitions aresmooth and continuous in accordance with progression from one end of therange covered by the oscillators to the other end of the range.

My invention also results in a substantial reduction of intermodulationdistortion. There are two types of distortion which commonly occur inelectronic organs. One type occurs because of non-linearities in theoscillator output circuits. This is well understood in the art as beinga source of intermodul-ation because of the paths which exist from eachoscillator to all the other oscillat-ors. It is evident that theattenuative collector bus 13 provides more isolation between oscillatorsthan is obtained by the usual type of collector bus. The other type ofdistortion is the Doppler modulation of high frequency tones which areradiated by loudspeakers that are simultaneously radiating highamplitude low frequency tones. It is evident from the previousdiscussion of my invention that the tones generated by the highestfrequency oscillators are attenuated over 20 decibels by the collectorbus, so that they are not radiated strongly by the loudspeaker carryingmost of the power of tones generated by the low frequency oscillators.The converse is also true. However, it is an important feature of myinvention that harmonics of the fundamental signal produced by anyoscillator are radiated in. the correct proportion by the loudspeaker orloudspeakers carrying the bulk of the energy from that oscillator. Thisis because of the lack of any frequency discrimination in theattenuative collector bus. Conventional woofer-tweeter speakerarrangements result in a perceptible tonal discontinuity as a result ofhaving the fundamental tones radiated from one loudspeaker and anapparently disconnected series of harmonics radiated from another.

In some cases, it may be desirable to connect two sets of oscillators tothe same attenuative collector bus. For example, it may be consideredadvantageous to have both sets of oscillators which constitute a mixtureor a celeste to be connected to the same bus. The second set ofoscillators 10 would 'be connected to the bus 13 through isolatingresistors 12 at the points 31, 35, 36, etc. in FIG. 1. Alternatively, itmay be desirable to provide each set of oscillators 10 with its ownattenuative collect-or bus and to use only the amplifiers andloudspeakers in common with both combinations of oscillators andcollector buses. In this case, the points 36, 37, and 38 in FIG. 1 wouldbe appropriate to establish the common connections. Sharing of theamplifiers and loudspeakers could also be on an either-or basis with thedesired circuit transfers being made at points 36', 37, and 38 onFIG. 1. 7

All of the preceding discussions have been centered around the chromaticarrangement which I prefer, that is, the oscillators 10 are connected tothe attenuative collector bus 13 in the order of their operatingfrequencies starting with the lowest frequency oscillator beingconnected to one end of the bus and ending with the highest frequencyoscillator being connected to the other end of the bus. The invention isalso applicable to other arrangements such as the one in which theseventy three oscillators are numbered in series and starting withnumber one, so that all odd-numbered oscillators are connected in orderof frequency to one half of the bus and all evennumbered oscillators areconnected in the reverse order of progression, according to frequency,proceeding toward the other end of the bus. For the purpose ofexplaining the circuit action, therefore, the oscillator tone sourcesshown in FIG. 1 may be arbitrarily numbered from left to right and it isunderstood that any desired sequence can be used.

One reason the chromatic arrangement is preferred is that it is the mostflexible for a given number of amplifiers and loudspeakers. To securethe same capability for adjustment of the low, middle, and highfrequency ranges in the divided arrangement, having odd-numbered sourceson one end and even-numbered sources on the other, it would be necessaryto have a five-channel system instead of the three channels required forthe chromatic arrangement. However, it is within the scope of myinvention to employ this arrangement by connecting another amplifier andloudspeaker channel to an intermediate point on eachhalf of theattenuative collector bus.

My invention is readily adaptable to many well known systems of tonegeneration, such as the one disclosed in my United States Patent No.2,941,435. The sine wave and harmonic containing buses disclosed in thatpatent can be made attenuative according to the present invention andthe number of amplifiers and loudspeakers can be increased as requiredto achieve a spatial distribution of sound according to the inventiondisclosed herein.

Others may readily adapt my invention to other conditions of use, suchas an electronic piano, or the like. As various changes may be made inthe form, construction, and arrangement of the parts herein, withoutdeparting from the spirit and scope of the invention and withoutsacrificing any of its advantages, it is to be understood that allmatters are to be interpreted as illustrative and not in any limitingsense.

I claim:

1. In an electronic musical instrument, means for producing a spatialdistribution of tones in accordance with their fundamental frequenciescomprising, in combination, at leastthree sources of oscillations ofvarious frequencies having desired tonal characteristics, an attenuativecollector bus, an isolating resistor for each of said oscillationsources having one end connected to its associated oscillation sourceand its other end connected to said collector bus, a plurality ofresistors connected in series in said collector bus, each of saidseries-connected resistors being disposed between collector busconnections of each pair of said isolating resistors, a plurality ofterminating resistors each having one end connected to said collectorbus and its other end connected to a point of common potential, at leastthree amplifiers each having its input connected to said collector busat juncture of one of said terminating resistors and said collector bus,a loudspeaker connected to output of each of said amplifiers, andcontrol means for said amplifiers.

2. In an electronic musical instrument according to claim 1, whereinsaid sources of oscillations are oscillators.

3. In an electronic musical instrument according to claim 1, wherein theinput of one of said amplifiers is connected to one end of saidcollector bus, the input of a second one of said amplifiers is connectedto the other end of said collector bus, and the input of a third one ofsaid amplifiers is connected intermediate the two ends of said collectorbus.

4. In an electronic musical instrument according to claim 1, whereinsaid plurality of terminating resistances constitute at least threeterminating resistors.

5. In an electronic musical instrument according claim 1, wherein one ofsaid terminating resistors connected to one end of said collector bus,another said terminating resistors is connected to the other end of saidcollector bus, and still another one of said terminating resistors isconnected intermediate the ends of said collector bus.

6. In an electronic musical instrument according to claim 1, whereinsaid sources of electrical oscillations are connected to points alongsaid attenuative collector bus in a sequential progression starting withthe source of electrical oscilltaions having the lowest fundamentalfrequency being connected to one end of said attenuative collector busand ending with the source of electrical oscillations having the highestfundamental frequency being connected to the other end of saidattenuative collector bus.

7. In an electronic musical instrument according to claim 1, whereinodd-numbered sources of said electrical oscillations are connected tosaid attenuative collector bus in a sequential progression starting withthe odd-numbered source having the highest fundamental frequency beingconnected to an approximate mid-point of said attenuative collector busand ending with the odd-numi is bered source having the lowestfundamental frequency being connected to one end of said attenuativecollector bus, and even-numbered sources of said electrical oscillationsare connected to said attenuative collector bus starting with theeven-numbered source having the highest fundamental frequency beingconnected to an approximate mid-point of said attenuative collector busand ending with the even-numbered source having the lowest fundamentalfrequency being connected to the other end of said attenuative collectorbus.

8. In an electronic musical instrument according to claim 1, whereinodd-numbered sources of said electrical oscillations are connected tostaid attenuative collector bus in a sequential progression startingwith the oddnumbered source having the lowest fundamental frequencybeing connected to an approximate mid-point of said attenuativecollector bus and ending with the oddnumbered source having the highestfundamental frequency being connected to one end of said attenuativecollector bus, and even-numbered sources of said electrical oscillationsare connected to said attenuative collector bus starting with theeven-numbered source having the lowest fundamental frequency beingconnected to an approximate mid-point of said attenuative collector busand ending with the even-numbered source having the highest fundamentalfrequency being connected to the other end of said attenuative collectorbus.

9. In an electronic musical instrument, means for producing a spatialdistribution of tones in accordance with their fundamental frequenciescomprising, in combination, a plurality of sources of electricaloscillations of various frequencies having desired tonalcharacteristics, an attenuative collector bus, an isloating resistor foreach of said oscillation sources having one end connected to itsassociated oscillation source and its other end connected to saidcollector bus, a plurality of resistors connected in series in saidcollector bus, each of said seriesconnected resistors being disposedbetween collector bus connections of each pair of said isloatingresistors, at least three terminating resistors each having one endconnected to said collector bus and its other end to a point of commonpotential, at least three amplifiers each having its input connected tosaid collector bus at juncture of one of said terminating resistors andsaid collector bus, and at least three loudspeakers, at least one ofsaid loudspeakers being connected to the output of each of saidamplifiers, each of said sources of electrical oscillations beingconnected respectively to a different point on said attenuativecollector bus through one of said isolating resistors, one of saidterminating resistors being connected to one end of said attenuativecollector bus, another of said terminating resistors being connected tothe other end of said attenuative collector bus, and at least one otherof said terminating resistors being connected intermediate the two endsof said attenuative collector bus, the input of one of said amplifiersbeing connected to one end of said attenuative collector bus, the inputof another of said amplifiers being connected to the other end of saidattenuative collector bus, and the input of at least one other of saidamplifiers being connected intermediate the two ends of said attenuativecollector bus.

10. In an electronic musical instrument according to,

claim 9, wherein said sources of electrical oscillations are normallyinoperative but are made operative as required for the performance of amusical composition by application of operating potentials by means ofcircuits including manually operated key contacts.

11. In an electronic musical instrument according to claim 9, whereinsaid sources of electrical oscillations are normally operative and theconnections to said attenuative collector bus are established by meansof circuits including manually operated key contacts.

12. In an electronic musical instrument according to claim 9, whereinthe gain of said amplifiers is varied simultaneously by common controlmeans to a predetermined degree for each of said amplifiers wherebycompensation is introduced for the variation in tonal sensitivity of thehuman ear with changes in dynamic level.

13. In an electronic musical instrument according to claim 9, whereinsaid three loudspeakers are physically disposed in a row with theloudspeaker at one end of the row being connected to the amplifiersource having the lowest frequency, and the loudspeaker at the other endof the row being connected to the amplifier source having the highestfrequency.

References Cited UNITED STATES PATENTS ARTHUR GAUSS, Primary Examiner.

10 D. D. FORRER, Assistant Examiner.

1. IN AN ELECTRONIC MUSICAL INSTRUMENT, MEANS FOR PRODUCING A SPATIAL DISTRIBUTION OF TONES IN ACCORDANCE WITH THEIR FUNDAMENTAL FREQUENCIES COMPRISING, IN COMBINATION, AT LEAST THREE SOURCES OF OSCILLATIONS OF VARIOUS FREQUENCIES HAVING DESIRED TONAL CHARACTERISTICS, AN ATTENUATIVE COLLECTOR BUS, AN ISOLATING RESISTOR FOR EACH OF SAID OSCILLATION SOURCES HAVING ONE END CONNECTED TO IS ASSOCIATED OSCILLATION SOURCE AND ITS OTHER END CONNECTED TO SAID COLLECTOR BUS, A PLURALITY OF RESISTORS CONNECTED IN SERIES IN SAID COLLECTOR BUS, EACH OF SAID SERIES-CONNECTED RESISTORS BEING DISPOSED BETWEEN COLLECTOR BUS CONNECTIONS OF SAID PAIR OF SAID ISOLATING RESISTORS, A PLURALITY OF 